Airliners remain vulnerable to easily obtained shoulder-fired missiles, while improper screening of air and sea cargo allows for potential smuggling of biological, chemical or nuclear weapons into the country, critics said. Meanwhile, the department has an understaffed intelligence unit and has no comprehensive strategy to defend the United States against various terrorist scenarios, lawmakers and security analysts said.There are fewer than 100 U.S. inspectors assigned overseas to inspect millions of cargo containers heading from foreign ports to the United States, according to a 135-page report on the department from Democrats in the House of Representatives.New York University Professor Paul Light, who studies government bureaucracies, noted organizational weaknesses in the department.Theyre facing some organizational problems, he said. Congress wants to build this department on the cheap; its like lashing together two mobile homes and putting them in the path of a hurricane, he added, referring to the 22 agencies and 180,000 employees that were merged to form the department last year.